A seat belt apparatus for restraining a vehicle occupant is equipped with a webbing take-up device. Some webbing take-up devices have been proposed which each include a so-called tension reducer mechanism for mitigating or eliminating an excessive sensation of pressure that the vehicle occupant feels when a webbing is applied to the occupant, or a pretensioner mechanism that eliminates a slight looseness called “slack” or the like by taking up a webbing belt onto a take-up shaft by a certain amount at the time when a vehicle rapidly decelerates or the like and that increases a restraining force by which the webbing belt restrains the body of the vehicle occupant, thereby allowing the body of the vehicle occupant to be held more reliably. Further, there also has been known a so-called motor retractor having a structure in which the aforementioned various functions are performed by a motor (by way of example, refer to the patent document 1 and the patent document 2 shown below).
In this type of motor retractor, for example, respective functions of the tension reducer and pretensioner as mentioned above can be not only demonstrated, but also an auxiliary operation for taking up or pulling out a webbing when the webbing is normally applied can be performed. Thus, an extremely beneficial effect is obtained.
Further, particularly in recent years, there has been considered, in the aforementioned motor retractor, a structure which detects a distance forward to another vehicle, an obstacle or the like using a forward looking monitoring device such as a distance sensor, and when the distance forward to the vehicle, an obstacle or the like is less than a certain value, activates a motor, and rotates a take-up shaft in a take-up direction by rotating force of the motor. In this type of motor retractor, a clutch is interposed between an output shaft of the motor and the take-up shaft so as to transmit rotating force of the motor to the take-up shaft and prevent transmission of rotation from the take-up shaft to the motor, and this clutch is structured to allow only the rotation from the output shaft of the motor to be transmitted to the take-up shaft.
Incidentally, in the conventional motor retractor as described above, in a case in which a driving force transmission mechanism that allows the rotating force of the motor to be transmitted to the clutch is comprised of a gear wheel or the like, if the motor is rapidly rotated so as to forcibly rotate the take-up shaft in the take-up direction, a large thrust force or reaction force acts on a case having the clutch accommodated and held therein, or a holding component for holding a gear mechanism, and the like. For this reason, it is necessary to improve extra strength of structural components such as a case, a cover and the like so that these components can resist such large thrust force or reaction force. This results in that realization of a light-weight device or cost reduction of the device would be impeded.    Patent document 1: JP-A No. 2001-130376    Patent document 2: JP-A No. 2001-347923